1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst adapted, for example, to be mounted in an exhaust pipe of a vehicle, and more specifically to an exhaust gas purifying catalyst comprising a catalyst layer formed on a surface of a honeycomb-shaped substrate in a manner that it contains a composite oxide which includes cerium (Ce) and zirconium (Zr) and has a hollow structure, and a catalytic metal supported by the composite oxide.
2. Description of the Related Art
As is commonly known, an air-fuel ratio of exhaust gas discharged from an engine is varied depending on engine operation states, such as acceleration, deceleration and steady states, and, in general, hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in the exhaust gas can be simultaneously converted only in a certain limited range around a theoretical air-fuel ratio of 14.7. As one technique of allowing an air-fuel ratio during acceleration or deceleration to fall with the purifying range, there has been known a technique of incorporating an oxygen-absorbing material into an exhaust gas purifying catalyst. This oxygen-absorbing material is operable, when oxygen in exhaust gas is in an excess state (lean air-fuel ratio), to absorb and store oxygen, and, when oxygen in exhaust gas is in a deficient state (rich air-fuel ratio), to release the oxygen stored therein, so as to allow the lean and rich air-fuel ratios to fall within the conversion range.
CeO2 (ceria) is known as one of the oxygen-absorbing materials. If CeO2 is used by itself, a specific surface area thereof is reduced when being exposed to heat of exhaust gas, and an oxygen absorbing/releasing capability will deteriorate due to change in properties thereof. For this reason, a composite oxide, such as CeZr-based composite oxide containing ZrO2 (zirconia), is known, and a catalytic metal is often supported by a carrier made of such a composite oxide.
As with the oxygen-absorbing material, it is also known to employ alumina as an oxide carrier for supporting a catalytic metal. Alumina is known as an oxide carrier generally having a higher thermal resistance and a larger specific surface area than those of the oxygen-absorbing material. While alumina has no oxygen absorbing/releasing capability, it excels in being able to support a catalytic metal in a highly dispersed state. With a view to further increasing the specific surface area of alumina having the above properties, a technique of forming alumina to have a hollow structure is disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 11-314035 and 2001-347167. Specifically, the Japanese Patent Laid-Open Publication No. 11-314035 discloses a technique of spraying a water/oil (W/O) emulsion solution comprising a primary component of aluminum (Al) and containing an element belonging to Groups IIa to VIIa and IIb in the periodic table, into a furnace chamber, and burning it therein at 900° C. or less to obtain an oxide carrier with a hollow structure having an outer diameter of 20 to 2000 nm and a shell thickness of several ten nm. The Japanese Patent Laid-Open Publication No. 2001-347167 discloses a technique of spraying a W/O emulsion solution comprising a primary component of Al and containing a rear-earth metal, and burning it at 1000° C. or less (preferably in the range of 650 to 950° C.) to obtain an oxide carrier with a hollow structure having an outer diameter of 20 to 2000 nm and a shell thickness of 50 nm or less.
As mentioned above, alumina has no oxygen absorbing/releasing capability. Thus, even if cerium (Ce) as rare earth metal is combined with alumina, a sufficient oxygen absorbing/releasing capability cannot be obtained. Form this point of view, the inventors conceived a CeZr composite oxide having a hollow structure without containing alumina. However, the CeZr composite oxide originally has a relatively small specific surface area and poor thermal resistance. Thus, even if the CeZr composite oxide is formed to have a hollow structure, the hollow structure will be destroyed after being exposed to heat of exhaust gas, to cause a problem about a significant decrease in specific surface area and a difficulty in ensuring adequate conversion performance.